Over the past forty years the development of synthetic blood substitutes has attracted increasingly serious interest from the medical community. This once fanciful idea received increasing credence in the early 1980s as the dangers of HIV transmission by blood transfusions become apparent. As a consequence, blood screening technology has become increasingly sophisticated and the chances of exposure to HIV from a blood transfusion dropped from 1 in 5,000 in the early 1980s to about 1 in 600,000 today. Nonetheless, there are still a variety of problems and risks with banked blood that are expected to persist in the foreseeable future.
Development of a successful blood substitute would eliminate the need to match blood types when transfusing trauma victims, as well as the risk that HIV, hepatitis or other pathogens could be transmitted by transfusion. The emergence of new viral threats such as SARS and “bird flu” illustrate the ongoing vulnerability of the world-wide blood supply to new and/or under-appreciated diseases for which reliable screening technology may not yet exist. In addition, a ready supply of synthetic blood substitute would also stretch natural blood supplies, which often run short around the holidays. For example, as recently as 2000 almost 10 percent of U.S. hospitals reported that they had to postpone surgeries because of unanticipated blood shortages. Finally, even when safe from contamination, blood transfusions can have negative effects on the immune system of the recipient.
Although blood is responsible for a multitude of functions in the body, a synthetic blood substitute would serve only to replicate its gas transporting function in which fresh oxygen is carried to cells and organs, exchanged there for carbon dioxide, and carbon dioxide is then removed. These are the most immediate and critical functions of blood, particularly in cases where blood is lost through massive trauma.
Two main approaches to such synthetic blood substitutes have been pursued over the past three decades: modified hemoglobin from human, animal, or recombinant origin and fluorocarbon emulsions. The hemoglobin approach is appealing because it most directly mimics the mechanism of oxygen transport in natural blood. However, when hemoglobin is not contained within a red blood cell, it is actually toxic to humans and most animals. Hence, much research on artificial hemoglobin focuses on reducing its toxicity through genetic engineering, chemical crosslinking, or attaching synthetic protective coatings that mimic the function of red blood cells. Although these approaches can ameliorate the issue of acute toxicity, hemoglobin-based blood substitutes also scavenge nitric oxide, leading to unwanted vasoconstriction and, hence, increased systemic and pulmonary artery pressure.
In the fluorocarbon approach, attempts to closely mimic natural blood are abandoned in favor of a purely synthetic mode of oxygen transport. In living systems, hemoglobin delivers oxygen in response to the cofactor 2,3-diphosphoglycerate (2,3-DPG). In contrast, the mechanism for oxygen delivery and carbon dioxide removal with fluorocarbons is simply a response to differences in the partial pressures of these gasses.
Attractive forces between fluorocarbon molecules are known to be particularly weak, and because of this, gas molecules are able to occupy molecular cavities between them, leading to the observed high gas solubilities. In general, gas solubilities in fluorocarbons follow the order of He<H2<N2<CO<O2<<CO2, where gases with the largest molecular volumes possess the highest solubilities. Fluorocarbons are also generally highly inert and nontoxic.
Because fluorocarbons are very hydrophobic, they tend to have desirably short residence times in various organs of the body. However, their extreme hydrophobicity is also a problem because blood plasma is an aqueous system that does not dissolve hydrophobic solutes. This problem has been addressed for many years through encapsulation of liquid fluorocarbons in a variety of sophisticated emulsion systems, which help to keep liquid fluorocarbons dispersed in aqueous media like blood plasma for, in some cases, very long periods of time. This is accomplished through use of sophisticated mixed fluorocarbon-hydrocarbon surfactants. Biocompatibility of these is optimized through judicious choice of the hydrophilic “head-group.” Particle sizes in the best emulsions can be as small as 0.1 μm. Nevertheless, choosing the right fluorocarbon and emulsion formulation is a fine art that balances concerns about oxygen solubility, fluorocarbon volatility, emulsion size, emulsion stability and residence time in various organs of the body. Hence, the vast majority of research and development activities on fluorocarbon-based blood substitutes has focused on engineering improved surfactants and emulsion formulations.
Fluorocarbon emulsions are removed from the blood stream and eventually expelled from the body primarily in two major steps. In the first step, the fluorocarbons are taken up and stored in reticuloendothelial system (RES) organs such as the liver, spleen and bone marrow. The rate of uptake in this step is strongly dependent on emulsion stability, droplet size, and hence, choice of emulsifying agent (surfactant). As an example, it was found that one particular egg yolk phospholipid (EYP)-coated fluorocarbon emulsion had an intravascular half-life (t1/2) in rats of 14 hours for a 2.7 g/kg dose with average droplet diameter of 0.09 μm. Surface modification using other surfactants than EYP led to shorter t1/2.
In the second step, which is much slower than the first, the fluorocarbons are released from the RES back into circulation, where they are taken up by circulating lipid carriers (lipoproteins and chylomicrons) and moved to the lungs for excretion through the lung alveoli into expired air. The excretion rate of the latter step strongly depends on the fluorocarbon molecular weight and lipophilicity.
Accordingly, there is a need for improved synthetic oxygen carriers for biomedical and biotechnical applications, which exhibit reduced toxicity, improved temperature stability, improved shelf life, and which overcome the various problems associated with the development of stable microemulsions.